"Dad, is there something we can build to measure the speed of a B-Daman marble?" 

The unit consists of a scrap board upon which we built a small firing range for the B-Daman. A marble travelling along its course breaks a light beam at the top of the run, signaling the Arduino via a photoresistor to start a stopwatch. It then travels a measured distance of 200 mms and hits a slam bar made of a popsicle stick, braced by LEGO, triggering a press-switch. The Arduino calculates the time between the light beam being broken and the switch being pressed and converts this millimeter per millisecond reading to kilometers per hour. The calculation can be displayed in the serial port of the Arduino's parent computer or on an LCD readout for stand-alone, 9-volt-battery-powered play.